This invention relates to epoxy resin compositions. In a specific embodiment, the invention relates to flame-retardant, monomer-diluted epoxy resin laminating compositions.
For use in applications such as electrical laminating, it is necessary to impart flame retardance to epoxy resins. Present techniques for imparting flame retardance to epoxy resins generally have one or more disadvantages. One method involves preparation of an epoxy resin by fusing a preformed liquid epoxy resin with tetrabromobisphenol-A. This technique, however, produces a resin which is solid at room temperature and must be diluted with large quantities of an organic solvent in order to reduce its viscosity to a level sufficient for impregnation of reinforcing fibers. The necessity for removal of this solvent causes a number of processing difficulties in the manufacture of fiber-reinforced composites for printed circuit boards. In compositions in which at least a portion of the organic solvent is replaced with a reactive diluent, there is a limit on the amount of the diluent which can be added without sacrificing laminating properties, since the diluent is not removed during the laminate preparation process.
The amount of solvent required to achieve acceptable viscosity can be reduced and the glass transition temperature of the cured laminate increased by reducing the molecular weight of the fusion resin product. However, this results in a lower bromine content in the resin. Commercial flame-retardant epoxy resins prepared by the fusion reaction of nonbrominated epoxy resins with tetrabromobisphenol-A generally have a molecular weight of 800 to 1000 and a bromine content of 18-21%. This bromine content is sufficient to provide flame retardance to standard commercial electrical laminates, which are cured with dicyandiamide, a curing agent used at a very low ratio (less than 5% by weight) of curing agent to epoxy resin.
When these resins are cured with curing agents, such as cycloaliphatic anhydrides, which are commonly used at higher ratios of curing agent to resin, however, the cured composition does not contain enough bromine to provide flame retardance unless the curing agent is also brominated. Also, the need for solvent removal could be avoided by using, as solvents for the resin, monomers such as styrene or acrylics, which polymerize during cure of the resin. However, the use of substantial amounts of the monomers for viscosity reduction would likewise lower the bromine content in the cured laminate to levels too low to provide the desired flame retardance.
Increasing the bromine content of the epoxy resin without increasing the molecular weight would thus appear to provide flame-retardance, retain the freedom to use non-brominated curing agents and diluent monomers, and minimize the volume of solvent or monomer necessary to provide low viscosity.
One way to prepare such epoxy resins with higher bromine content would be the direct glycidation of tetrabromobisphenol-A with epichlorohydrin, producing a low molecular weight epoxy resin not containing any units derived from non-brominated bisphenols, as would be the case with a fusion product. Such low molecular weight epoxy resins consisting primarily of the diglycidyl ether of tetrabromobisphenol-A are available as amorphous solids containing about 45% bromine. They are easily dissolved in styrene or other monomers to provide low-viscosity solutions of high bromine content. These solutions can be cured with various types of epoxy curing agents, including aromatic amines, to give flame-retardant cured epoxy resins of high glass transition temperature.
It has been found, however, that monomer solutions containing large fractions of the diglycidyl ether of tetrabromobisphenol-A can be subject to crystallization with storage. Redissolving the material requires heat and agitation, an inconvenience for users. Heating solutions containing reactive monomers to dissolve crystals may also cause polymerization and gelation of the solutions.
It is therefore an object of the invention to provide a monomer-diluted, flame-retardant, low molecular weight epoxy composition which has improved storage stability.